The Pennsylvania State University The Graduate School Department of Materials Science and Engineering APPLICATION OF POURBAIX DIAGRAMS IN THE HYDROMETALLURGICAL PROCESSING OF BASTNASITE A Thesis in Materials Science and Engineering by Isehaq S. Al-Nafai © 2015 Isehaq S. Al-Nafai Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science May 2015 i The thesis of Isehaq S. Al-Nafai was reviewed and approved* by the following: Kwadwo Osseo-Asare Distinguished Professor of Materials Science and Engineering, Metallurgy and Energy and Geo-Environmental Engineering. Thesis Adviser Hojong Kim Assistant Professor of Materials Science and Engineering Ismaila Dabo Assistant Professor of Materials Science and Engineering Suzanne Mohney Professor of Materials Science and Engineering and Electrical Engineering Chair, Intercollege Graduate Degree Program in Materials Science and Engineering *Signatures are on file in the Graduate School. ii ABSTRACT The hydrometallurgical processing of bastnasite was studied by constructing Pourbaix (potential vs. pH) diagrams at room temperature using HSC Chemistry 5.0 software. Different systems were considered to understand the overall behaviors of bastnasite and its species in aqueous systems. Most of the thermodynamic data were taken from HSC database, others were collected from literature and some were estimated. The standard Gibbs free energy of formation of bastnasite, REFCO3, were estimated using four different estimation methods. Each method shows its applicability, and the obtained average values were -379.9 kcal/mol for CeFCO3, -382.3 kcal/mol for LaFCO3, -379.8 kcal/mol for NdFCO3 and -381.3 kcal/mol for PrFCO3. RE-F-CO3-H2O systems show the stability regions of bastnasite which are located in nearly neutral to alkaline media (pH ~ 6.5-11). The RE-F-CO3-(SO4)-(Cl)-(NO3)-H2O systems were considered to display the decomposition behaviors of bastnasite when treated by concentrated acid solutions. Furthermore, the decomposition behavior of bastnasite by alkaline solutions can be explained by these systems. In treatment with sulfuric acid, -2 CeFCO3 decomposes at pH ~ 6.2 when {SO4 } = 1 m while other REFCO3 decomposes at pH ~ 8. On the other hand, hydrochloric and nitric acids decompose bastnasite at pH ~ 2. The alkaline decomposition of REFCO3 is feasible at pH ~ 11. In the investigation of recovery and recycling of rare earths by precipitation, the systems RE-C2O4-H2O were considered which show the large stability regions of RE oxalate hydrates over the pH range (-2 – 11). All these trends, which revealed by these systems, were related to the hydrometallurgical processing of bastnasite. iii Table of Contents Page List of Tables………………………………………………………………. vii List of Figures……………………………………………………………… viii List of Abbreviations………………………………………………………. xi Acknowledgements………………………………………………………… xii Chapter 1. General Introduction……………..………………………….. 1 1.1 Brief Background………………………………………………………. 1 1.2 Motivation……………………………………………………………… 2 1.3 Objectives……………………………………………………………… 3 1.4 Organization of the Thesis……………………………………………… 3 References………………………………………………………………….. 5 Chapter 2. Background and Literature Review………………………... 7 2.1 Rare Earth Elements……………………………………………………. 7 2.2 Rare Earth Minerals…………………………………………………….. 8 2.3 Applications of Rare Earth Elements…………………………………… 10 2.4 Bastnasite………………………………………………………………. 11 2.5 Extraction and Processing of Rare Earths……………………………… 13 2.6 Processing of Bastnasite………………………………………………... 14 2.7 Pourbaix Diagrams and their Applications……………………………... 17 2.8 Conclusions……………………………………………………………... 20 References…………………………………………………………………… 21 Chapter 3. Estimation of the Standard Free Energy of Formation of Bastnasite……………………………………………………………………. 26 3.1 Introduction……………………………………………………………… 26 3.2 Methods………………………………………………………………….. 27 3.2.1 Simple Thermodynamic Approach………..……………………….. 28 iv 3.2.2 Single Salts Approach.…………………………………………….. 29 3.2.3 Linear Free Energy Relationship……………..…..……………….. 31 o o 3.2.4 Linear ΔH f – ΔG f Relationship………………………………..… 33 3.3 Results and Discussion…………………………………………………... 34 3.3.1 Simple Thermodynamic Approach……………………….……….. 34 3.3.2 Single Salts Approach…………………………………………….. 36 3.3.3 Linear Free Energy Relationship …………….…………….…….. 37 o o 3.3.4 Linear ΔH f – ΔG f Relationship ………………….……………… 38 3.3.5 Eh-pH Diagrams and Test of the Estimated Data………………… 39 3.4 Conclusions………………………………………………………………. 42 References…………………………………………………………………… 43 Chapter 4. Cerium Systems……………………………………………….. 47 4.1 Introduction……………………………………………………………… 47 4.2 Methods………………………………………………………………….. 49 4.2.1 Thermodynamic Data…………………………………………….. 49 4.2.2 Hydrometallurgical Processing of Cerium Bastnasite……………. 50 4.2.3 Acid Treatment of Cerium Bastnasite…………………………….. 51 4.2.4 Alkali Treatment of Cerium Bastnasite…………………………... 52 4.2.5 Chemical Treatment of Cerium Bastnasite and the Eh-pH Diagrams 52 4.3 Results and Discussion…………………………………………………... 54 4.3.1 Ce-H2O System…………………………………………..……….. 54 4.3.2 Ce-F- H2O System……………………………….……………….. 56 4.3.3 Ce-CO3- H2O System……………………………………….…….. 58 4.3.4 Ce-F-CO3- H2O System…………………………………….…….. 60 4.3.5 Ce-SO4- H2O System……………………………….…………….. 64 4.3.6 Ce-F-CO3-SO4- H2O System…………………………….……….. 66 4.3.7 Ce-Cl- H2O System………………………………………………. 69 4.3.8 Ce-F-CO3-Cl- H2O System………………………………………. 71 4.3.9 Ce-NO3- H2O System…………………………………………….. 73 4.3.10 Ce-F-CO3-NO3- H2O System…………………………………… 75 v 4.3.11 Ce-C2O4- H2O System………………………………………….. 77 4.4 Conclusions………………………………………………………………. 79 References…………………………………………………………………… 81 Chapter 5. La-, Nd- and Pr- Systems……………………………………… 85 5.1 Introduction……………………………………………………………… 85 5.2 Methods …………………………………………………………………. 87 5.2.1 Thermodynamic Data …………………………………………….. 87 5.2.2 Chemical Processing of Bastnasite and Eh-pH Diagrams………… 87 5.3 Results and Discussion …………………………………………………… 90 5.3.1 RE- H2O Systems ………….………………………………………. 90 5.3.2 RE-CO3- H2O Systems ……………………………………………. 92 5.3.3 RE-F- H2O Systems ……………………………………………….. 96 5.3.4 RE-F-CO3- H2O Systems…………………………………………... 101 5.3.5 RE-SO4- H2O Systems …………………………………………….. 105 5.3.6 RE-F-CO3-SO4- H2O Systems……………………………………… 109 5.3.7 RE-Cl- H2O Systems….………………………….…………….…… 113 5.3.8 RE-F-CO3-Cl- H2O Systems………………..……………………… 117 5.3.9 RE-NO3- H2O Systems……………………………………………... 121 5.3.10 RE-F-CO3-NO3- H2O systems……………………………………. 125 5.3.11 RE-C2O4- H2O Systems…………………………………………… 129 5.4 Conclusions……………..…………………………………………………. 132 References……………………………………………………………………… 134 Chapter 6. General Conclusions and Suggestions for Future Work………. 137 6.1 General Conclusions…………………………..……………………………. 137 6.2 Suggestions for Future Work………………………...…………………....... 138 Bibliography……………………………………………………………….…… 139 vi List of Tables Table Page Table 2.1: Concentrations of REE in earth’s crust compared to some common elements in ppm……………………………………………………………………. 8 Table 2.2: Main rare earth minerals…………………………………………….. 10 Table 2.3: Applications of rare earth elements………………………………….. 12 o Table 3.1: Different methods used to estimate ΔG f of REFCO3…………………. 28 Table 3.2: Thermodynamic data used in single salt approach…………..…... 30 Table 3.3: Data used in the linear free energy relationship approach……….. 32 o o Table 3.4: Thermodynamic data used in the linear ΔH f – ΔG f relationship. 33 o Table 3.5: Standard free energy of formation (ΔG f,298) of some RE species in kJ/mol …………………………………………………………………….. 34 o Table 3.6: Summary of the estimated values of ΔG f of REFCO3 using different methods……...………………………………..……………………. 39 Table 3.7: Thermodynamic data used in Ce-F-CO3-H2O system diagram…. 40 Table 4.1: Thermodynamic data used for Ce-systems………………………. 50 Table 4.2: Eh-pH systems and their applications in cerium bastnasite hydrometallurgy……………………………………………………………… 53 - Table 5.1: Thermodynamic data for RE-F-CO3-(SO4)-(Cl)-(NO3 ) (C2O4)- o H2O systems at 25 C…………………………………………………………. 88 Table 5.2: Eh-pH systems and their applications in bastnasite hydrometallurgy……………………………………………………………… 89 vii List of Figures Figure Page Figure 2.1: Physical beneficiation of Mountain Pass bastnasite ore………... 15 Figure 2.2: Chemical treatment of bastnasite concentrate…………………. 16 Figure 2.3: Theoretical Eh-pH diagram for M-H2O system……………….. 18 o o Figure 3.1: Relation between ΔG f of La compounds vs. ΔG f of Ce compounds………………………………………………………………….. 35 o o Figure 3.2: Relation between ΔH f vs. ΔG f of several RE salts………………………………………………………………………….. 38 -3 Figure 3.3: Ce-F-CO3-H2O system, {Ce} = 10 m, {F} = {C} = 1.0 m…... 41 -3 Figure 3.4: Ce-F-CO3-H2O system, {Ce} = {F} = {C} = 10 m………….. 41 o Figure 4.1: Eh-pH diagrams for Ce-H2O system at 25 C………………….. 55 o Figure 4.2: Eh-pH diagrams for Ce-F-H2O system at 25 C………………… 57 o Figure 4.3: Eh-pH diagrams for Ce-CO3-H2O system at 25 C…………….. 59 o Figure 4.4: Eh-pH diagrams for Ce-CO3-F-H2O system at 25 C………….. 61,62 - Figure 4.5: A log S vs. log [F ] plot for CeF3(s)…………………………….. 63 o Figure 4.6: Eh-pH diagrams for Ce-SO4-H2O system at 25 C…………….. 65 o Figure 4.7: Eh-pH diagrams for Ce-CO3-F-SO4-H2O system at 25 C……… 67 o Figure 4.8: Eh-pH diagrams for Ce-Cl-H2O system at 25 C………………. 70 o Figure 4.9: Eh-pH diagrams for Ce-CO3-F-Cl-H2O system at 25 C……….. 72 o Figure 4.10: Eh-pH diagrams for Ce-NO3-H2O system at 25 C………….. 74 o Figure 4.11: Eh-pH diagrams for Ce-CO3-F-NO3-H2O system at 25 C……. 76 o Figure 4.12: Eh-pH diagrams for Ce-C2O4-H2O system at 25 C…………… 78 viii o Figure 5.1: Eh-pH diagrams for La-, Nd- and Pr-H2O systems at 25 C…... 91 o Figure 5.2: Eh-pH diagrams for La-CO3-H2O system at 25 C…………... 93 o Figure 5.3: Eh-pH diagrams for Nd-CO3-H2O system at 25 C……………. 94 o Figure 5.4: Eh-pH diagrams for Pr-CO3-H2O system at 25 C……………. 95 o Figure 5.5: Eh-pH diagrams for La-F-H2O system at 25 C………………. 97 o Figure 5.6: Eh-pH diagrams for Nd-F-H2O system at 25 C……………… 98 o Figure 5.7: Eh-pH diagrams for Pr-F-H2O system at 25 C……………….. 99 - Figure 5.8: A log S vs. log [F ] plot for LaF3(s)……………………………..